1. Field of the Invention
This invention relates to systems and methods for producing an audio signal in response to a digital time code on a motion picture film.
2. Description of the Related Art
Current motion picture films employ analog sound recording techniques. Most motion pictures use an analog optical sound track that is printed onto the film along with the picture, and is optically scanned to reproduce the sound. Another technique is to record the sound onto magnetic strips that are coated along the edges of the film; this process is used for example in producing "Dolby.RTM. 70 mm Sound".
Optical sound tracks have a number of shortcomings. They are susceptible to distortion caused in the laboratory printing process, have a limited frequency and amplitude range, and are subject to damage and noise caused by wear and dirt accumulation. Magnetically recorded sound tracks are subject to noise problems similar to those encountered with other audio tapes. It would be highly desirable to upgrade the quality of motion picture sound to a level comparable to that achieved with a compact disk, through the use of a digital sound system.
Several attempts have been made to achieve a digital movie sound system. One approach has been to replace the analog sound track directly with a digital track by recording digital sound data in the area previously occupied by the analog track on the film. Although theoretically this approach could be used to yield a higher quality sound reproduction, it has proven difficult for film laboratories to reliably print the small bit size required to fit the amount of digital data needed in the restricted area available. There would also be a fairly low limit to the number of sound tracks that could be encoded onto the film. As a practical matter, this type of sound encoding is incompatible with the analog sound reproduction equipment available in most theaters, and would require the theater to have special dedicated sound equipment for the digital sound tracks. Any theater that wanted to run films with this type of digitally encoded sound track would thus have to have two separate sets of sound equipment, if they also wanted to be able to play films with conventional analog sound tracks. Conversely, the distribution of a film with such a digital sound track would be likely to suffer because it could not be run in a theater that had not added a digital sound capability.
Another approach has been to print the digital information in "unused" areas of the motion picture print, such as between the film's sprocket holes. With this approach it is also difficult to obtain reliable prints from the laboratory and to have them remain reliable during extended use in theaters; the sprocket hole areas of a film are particularly subject to wear.
An alternate approach is to record the digital sound data on a separate medium such as a laser disk, and to synchronize the digital sound source with the picture. An implementation of this approach has been to print a time code onto the film along with a normal analog sound track, and to use the time code to directly access the digital sound storage medium. This method does not take into account the fact that short pieces of film are sometimes removed because of film damage, leaving "jumps" or "edits" in the picture that the sound track must follow. However, an instantaneous jump in the digital audio is not possible from most digital audio sources, such as laser disks. Furthermore, maintaining a mechanical synchronization of a laser disk to a film is difficult.
The related application Ser. Nos. 07/620,825 and 07/842,789 attempt to resolve this problem by transferring the digital audio data from a permanent storage medium, such as a digital audio tape (DAT) or laser disk, to an intermediate fast access digital memory, and to then transfer data from the intermediate memory for conversion to analog and playback in the theater as needed. This technique permits virtually instantaneous jumps to be made in the audio playback to match film edits, and also provides a simpler synchronization without the need for a constant and accurate mechanical synchronization of the digital sound with the picture. However, it has the disadvantage of limiting the size of edit jumps that can be accommodated to the size of the intermediate digital buffer memory. Large jumps require a large memory, which significantly increases the cost of the system.